SILICIC MAGMA ENTERING A BASALTIC MAGMA CHAMBER - ERUPTIVE DYNAMICS AND MAGMA MIXING - AN EXAMPLE FROM SALINA (AEOLIAN ISLANDS, SOUTHERN TYRRHENIAN SEA)

The Pollara tuff-ring resulted from two explosive eruptions whose depo sits are separated by a paleosol 13 Ka old. The oldest deposits (LPP, about 0.2 km3) consist of three main fall units (A, B, C) deposited fr om a subplinian column whose height (7-14 km) increased with time from A to C, as a consequence of the increased magma discharge rate during the eruption (1-8 x 10(6) kg/s). A highly variable juvenile populatio n characterizes the eruption. Black, dense, highly porphyritic, mafic ejecta (SiO2 = 50-55%) almost exclusively form A deposits, whereas gre y, mildly vesiculated, mildly porphyritic pumice (SiO2=56-67%) and whi te, highly vesiculated, nearly aphyric pumice (SiO2=66-71%) predominat e in B and C respectively. Mafic cumulates are abundant in A, while cr ystalline lithic ejecta first appear in B and increase upward. The LPP result from the emptying of an unusual and unstable, compositionally zoned, shallow magma chamber in which high density mafic melts capped low density salic ones. Evidence of the existence of a short crystal f ractionation series is found in the mafic rocks; the andesitic pumice results from complete blending between rhyolitic and variously fractio nated mafic melts (salic component up to 60 wt%), whereas bulk dacitic compositions mainly result from the presence of mafic xenocrysts with in rhyolitic glasses. Viscosity and composition-mixing diagrams show t hat blended liquids formed when the visosities of the two end members had close values. The following model is suggested: 1. A rhyolitic mag ma rising through the metamorphic basement entered a mafic magma chamb er whose outer portions were occupied by a highly viscous, mafic cryst al mush. 2. Under the pressure of the rhyolitic body the nearly rigid mush was pushed upwards and mafic melts were squeezed against the wall s of the chamber, beginning roof fracturing and mingling with silicic melts. 3. When the equilibrium temperature was reached between mafic a nd silicic melts, blended liquids rapidly formed. 4. When fractures re ached the surface, the eruption began by the ejection of the mafic mel ts and crystal mush (A), followed by the emission of variously mingled and blended magmas (B) and ended by the ejection of nearly unmixed rh yolitic magma (C).